1. Field of the Invention
The present invention relates to a solid electrolytic capacitor utilizing the so-called valve metal such as tantalum or niobium. In particular, the present invention relates to a solid electrolytic capacitor provided with a pair of leads to enable mounting onto a printed circuit board, for example.
2. Description of the Related Art
Typically, a solid electrolytic capacitor of the above-mentioned type comprises a capacitor element which includes a sintered porous chip and an anode wire projecting from an end surface of the chip. Such a porous chip is made of powdered valve metal such as tantalum or niobium, which is compacted into a rectangular solid or a circular column. The particles of the metal powder are covered by a highly insulating dielectric layer. The outer surface of the chip is formed with a solid electrolytic layer upon which a cathode layer is formed. The above-mentioned anode wire is connected to an anode lead, while the cathode layer on the chip is connected to a cathode lead. In use, the anode and cathode leads are soldered to e.g. a printed circuit board (see JP-A-2003-68576, for example).
In the above-described capacitor element, the solid electrolytic layer formed on the chip has a large contact area coming into contact with the dielectric layer, which is advantageous for providing a large capacitance per unit volume of the capacitor element. On the other hand, the solid electrolytic layer has an outer surface area (noncontact surface area) which does not come into contact with the dielectric layer. This noncontact surface area of the electrolytic layer is generally equal to the outer surface area of the sintered chip, but much smaller than the above-mentioned contact area contacting with the dielectric layer. Accordingly, the solid electrolytic layer has a considerably large equivalent series resistance (ESR).
As conventionally known, generally the equivalent series resistance is inversely proportional to the above-mentioned noncontact surface area of the electrolytic layer. Therefore, the noncontact surface area should be increased for decreasing the equivalent series resistance, thereby improving the capacitor performance in a high frequency range.
However, as previously noted, the porous chip of the capacitor element is a rectangular solid (having a rectangular cross section) or a circular column (having a circular cross section), and the noncontact surface area of the solid electrolytic layer is generally equal to the outer surface area of the chip. Therefore, to increase the noncontact surface area, the outer surface area of the chip needs to be increased, in other words, the external size of the chip needs to be increased. Under these circumstances, the conventional solid electrolytic capacitor tends to be unfavorably large in size and weight in order to improve the capacitor performance in a high frequency range.